11/21
XC9248
Series
■OPERATIONAL EXPLANATION (Continued)
<Thermal Shutdown>
For protection against heat damage of the ICs, thermal shutdown function monitors chip temperature. The thermal shutdown
circuit starts operating and the Nch MOS driver transistor will be turned off when the chips temperature reaches 150℃. The L
X
pin enters the C
L
discharged state and stops functioning at GND level (0V). When the temperature drops to 125℃ or less after
shutting of the current flow, the IC performs the soft-start function to initiate output startup operation.
<UVLO Circuit>
When the V
IN
voltage becomes 3.8V (TYP.) or lower, the Nch MOS driver transistor is forced OFF. The L
X
pin enters the C
L
discharged state and stops functioning at GND level (0V). When the V
IN
voltage becomes 3.9V (TYP.) or higher, switching
operation takes place. By releasing the UVLO function, the IC performs the soft-start function to initiate output startup operation.
The soft-start function operates even when the V
IN
voltage falls momentarily below the UVLO detect voltage. The UVLO circuit
does not cause a complete shutdown of the IC, but causes pulse output to be suspended; therefore, the internal circuitry
remains in operation.
<Bootstrap method>
An Nch MOS driver Tr. is used for the High side driver, and a voltage higher than the V
IN
voltage is needed to turn the driver on.
For that purpose, the bootstrap method is used to generate a voltage higher than the V
IN
voltage. The C
BST
capacitance is
connected between BST and LX, and because the V
LX
voltage is lower than the 4.6V (TYP.) V
L
voltage that is the internal power
supply, C
BST
is charged from V
L
.
<V
OUT
short-circuit protection>
With the A type, when the output voltage V
OUT
is shorted to GND or is near a shorted state (the FB voltage is1/2 or lower), and
a current over the current limit flows to the High side or Low side driver Tr., a V
OUT
short circuit is detected and the High side and
Low side driver Trs. are immediately turned off and latched. The L
X
pin enters the C
L
discharged state and stops functioning at
GND level (0V). Once in the latched state, operation is resumed by either turning the IC off and restarting with the EN pin, or by
re-input into the V
IN
pin (the voltage is lowered below the under-voltage lockout detection voltage once).
<L
X
short-circuit protection>
If the event that the L
X
pin shorts to GND, L
X
short-circuit protection activates for protection from over-current due to rush
current and to protect the IC.
If the L
X
pin shorts to GND, High side current limiting will activate due to rush current when the High side driver Tr. turns on.
The High side driver Tr. turn offs, and the Low side driver Tr. turns on at the same time. At this time, if Low side current limiting
did not activate, an L
X
short-circuit is detected, and the Low side driver is turned off and latched at the same time as the High
side driver Tr. Once in the latched state, operation is resumed by either turning the IC off and restarting with the EN pin, or by
re-input into the V
IN
pin (the voltage is lowered below the under-voltage lockout detection voltage once).
<V
OUT
over-voltage protection>
To minimize output voltage overshoot, V
OUT
over-voltage protection activates when V
OUT
overshoot occurs due to the output
resistance changing from a heavy load to a light load or otherwise. When V
OUT
overshoot occurs and the FB voltage that senses
V
OUT
rises to 0.9V (TYP.) or more, the High side driver Tr. is immediately turned off and the Low side driver Tr. is turned on to
prevent V
OUT
overshoot. When the FB voltage falls to 0.8V (TYP.) or less due to hysteresis, the High side driver Tr. turns on at
the next clock cycle.
